If I want to buy a used 3457A, is it better to buy one "as-is" and send it for calibration, or perhaps spend a bit more and get one already calibrated? Just one example: http://www.ebay.com/itm/HP-3457A-6-5-Digit-Lab-Quality-Front-Terms-Fully-Refurbished-NIST-Cal-/300928374155 Joe Gray W5JG